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The value of cos1(cos7π6) is equal to
(a) 7π6,
(b) 5π6,
(c) π3,
(d) π6.

Answer
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Hint: We start solving the problem by recalling the principal value or range of the inverse cosine function cos1(x). We then find the value of the cos(7π6) using the fact that cos(π+θ)=cosθ and substitute it in the given cos1(cos7π6). We then follow the range of inverse cosine function cos1(x) to find the value of cos1(cos7π6).

Complete step by step answer:
According to the problem, we need to find the value of cos1(cos7π6).
We know that the function cos1(x) has principal value or the range of cos1(x) is restricted between 0 and π. This means that cos1(cos7π6) has value between 0 and π -(1).
Let us solve for the value of cos1(cos7π6).
Now let us find the value of cos(7π6). We know that cos(π+θ)=cosθ.
cos(7π6)=cos(π+π6).
cos(7π6)=cos(π6).
cos(7π6)=32.
So, we have got cos1(cos7π6)=cos1(32).
We know that cos1(32)=5π6, following the definition of the principal value of cos1(x) as mentioned in equation (1).
So, we get cos1(cos7π6)=5π6.
We have found the value of cos1(cos7π6) as 5π6.

So, the correct answer is “Option b”.

Note: We should not directly say the value of cos1(cos7π6) as 7π6. This is because of the fact that the value 7π6 is not in the principal value of the function cos1(x). Whenever we get this type of problem, we should answer it in between principal values. If it is specified that the values outside the principal range are also allowed, then we can say all the possible answers. We don’t need to remember the hectic formulas related to inverse trigonometric functions while solving this type of problems.
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